Ice cover alters the behavior and stress level of brown trout Salmo trutta

Stress responses of juvenile brown trout under winter conditions in a laboratory stream

“…Ventilation frequency is a sensitive measure to perceived predation risk (Hawkins et al, 2004;Brown et al, 2005;Hawkins et al, 2007), and fish without overhead shelter increase their oxygen consumption presumably to increase predator vigilance (Millidine et al, 2006). Moreover, ventilation frequency increases rapidly as a response to a stressor (Barreto & Volpato, 2004) and seems largely unaffected by social interactions (Watz et al, 2015). Under experimental winter conditions, Watz et al (2013) showed that trout under full surface ice cover reduced their ventilation frequency and increased foraging activity, which might lead to an improved energetic balance (Watz et al, 2016).…”

“…In treatment (2), no surface ice or net were used. In treatment (3), a 6-cm-thick ice block (0.7 9 0.6 m), created by freezing distilled water in a plastic tray (Dilling, IKEA AB, Ä lmhult, Sweden), was suspended by concrete blocks near the surface (Watz et al, 2015), covering the deep section of the arena. Thus, in the treatment without ice (2) and in the surface ice treatment (3), trout had access to the instream wood shelter.…”

“…These measurements were recorded because previous studies have shown effect of stream ice on microhabitat selection and activity (Linnansaari & Cunjak, 2013;Watz et al, 2016). During this 10 min period, ventilation frequency was measured and eye and body colour were scored according to Watz et al (2015). In brief, eye colour was assessed from observations and the video recordings and scored on a scale from 1 to 5, where a score of 1 represents an unstressed trout with pale iris and sclera and a 5 represents a stressed trout with dark iris and sclera.…”

“…In brief, eye colour was assessed from observations and the video recordings and scored on a scale from 1 to 5, where a score of 1 represents an unstressed trout with pale iris and sclera and a 5 represents a stressed trout with dark iris and sclera. Body colour was scored similarly (1 = pale, unstressed; 5 = dark, stressed) (O'Connor et al, 1999;Suter & Huntingford, 2002;Watz et al, 2015). After the non-invasive measurements, trout were quickly hand-netted (\20 s), anesthetised with a lethal dose of benzocaine (completely anaesthetised in \60 s) and killed with a blow to the head.…”

“…Juvenile salmonids seem to cope relatively well with low temperatures and river ice formation per se (Linnansaari & Cunjak, 2013), but lack of access to shelters against predation from endothermic piscivores may lead to stress (Finstad et al, 2007;Watz et al, 2015) and has shown to be a major factor that influences overwinter mortality (Huusko et al, 2007;Brown et al, 2011).…”